The invention relates to a sprocket assembly for mounting of a driver on a rear wheel hub of a bicycle. The multiple sprocket assembly includes a cluster of sprockets of different sizes for engaging a chain and for transferring a force introduced by the chain to the hub, with the possibility of shifting the chain from one sprocket to another by means of a derailleur.
In some conventional multiple sprocket assemblies, a cassette or multiple sprocket cluster part with a plurality of separate sprockets with different diameters and numbers of chain teeth is arranged on a driver of a hub via a spline-type interface. The teeth of each sprocket engage alternatingly with the chain upon actuation of the derailleur and transfer a torque from the pedaling forces of the rider to the driver via an inner profile that connects the sprockets to the driver splines. Each of these sprockets therefore individually transfers the torque to the driver, as a result of which each sprocket must be stronger and heavier both in the area of the profile of the sprocket and in the sprocket arms due to the lateral forces as a result of the usually misaligned chain.
To meet different requirements, for example in cycling, as designs become lighter and the demand for precision becomes greater in conjunction with a further increase in the number of gears, the basic approach of constructing multiple sprockets in a single piece was achieved.
For instance, such a multiple sprocket arrangement is known from EP 1 972 539 in which the sprockets are arranged in a conical support structure in which tubular sections and disc-shaped sections alternate in a stepped manner and teeth arranged on the periphery thereof are designed to engage with the roller chain. The support structure can be embodied with very thin walls, the minimum thickness of which is defined above all by the manufacturing tolerances.
In multiple sprocket arrangements comprised of individual sprockets, the individual sprockets are each non-rotatably connected to the driver. A torque-transferring profile, typically in the form of castellations or projections, is located on the inside of the each sprocket and creates a positive connection between the individual sprocket and the driver. Moreover, several adjacent individual sprockets can be joined together into a sprocket cluster by screwing them together with spacer rings positioned therebetween to ensure that a defined spacing is maintained in the axial direction between the individual sprockets. This makes it easier to mount a connected cluster of sprockets onto the driver, which is necessary in the event of tooth wear on the sprockets and permitting replacement with new sprockets. The smallest individual sprocket is given a special task to secure attachment of the cluster to the driver.
For the axial fixing of the sprocket cluster, an attachment screw is typically used which is screwed with its external thread into an internal thread on the driver. Tool surfaces on the attachment screw enable engagement of a tool on same in order to apply torque to the attachment screw to tighten or loosen it.
During pedaling operation, the driver and the sprocket cluster rotate under and through the chain, which engages on the sprocket at a constant angle of inclination, and the point of applied force of the tensile force of the chain on the sprocket cluster changes constantly at the interconnection of driver and sprocket.
The internal diameter of the sprocket cluster is greater than the external diameter of the driver so that it is possible to mount the sprocket cluster by pushing it axially onto the driver.
When the sprocket cluster undesirably rotates with respect to the driver, the sprocket cluster therefore rolls off on the outer circumferential surface of the driver (i.e., slippage) and, in doing so, tends to rotate about a lesser angle than the driver. As a result of the restricted rotation between sprocket cluster and driver through engaging projections on the sprocket cluster, a back-and-forth flexing movement of the sprocket cluster with respect to the driver occurs. This undesirable flexing movement can contribute to the loosening of the attachment screw, causing the attachment screw to come unscrewed, resulting in the loss of a secure connection between sprocket cluster and driver. As a countermeasure, knurled elements both on the attachment screw and on conventional smallest sprockets have proven expedient. The knurled elements are arranged with a radial extension in relation to the axis of rotation of the sprocket and engage each other as a result of the axial abutment between an abutment on the driver on the one hand and the head of the attachment screw on the other hand. If a flexing movement of the smallest sprocket now occurs with respect to the driver within the limits predetermined cooperating knurled elements, the attachment screw can no longer loosen. Rather, it moves together with the back-and-forth movement of the smallest sprocket during the abovementioned flexing movement, because the knurling prevents greater movement than that between the smallest sprocket and the attachment screw.
The individual knurled elements are aligned radially to inhibit loosening rotation between the smallest sprocket and the attachment screw. In the case of a smallest sprocket that is manufactured by means of punching and reshaping, the shaped elements are stamped in as well, for which purpose complementary shaping elements are present on the corresponding stamping tools. The production of the shaped elements occurs incidentally as part of the tool that forms the overall sprocket, as it were, without carrying out additional processing steps that might have a negative impact on manufacturing costs.
The situation is different with sprocket clusters that are produced by means of manufacturing methods involving machining. In that case, shaping work steps are first performed through lathe machining steps and subsequent milling operations in which the shaped elements of the knurled teeth cannot be produced in an incidental manner. Rather, other additional processing steps would have to be performed, with the associated additional processing times and costs. It is especially disadvantageous here that the shaped elements of the knurled teeth, due to their radial alignment, are not conducive to a milling operation, much less to lathe machining.